What is a Slavic fermented cereal-based non-alcoholic or low alcoholic beverage called?

Kvass is a fermented cereal-based non-alcoholic or low alcoholic (0.5–1.0% or 1–2 proof) beverage with a slightly cloudy appearance, light-dark brown colour and sweet-sour taste. It may be flavoured with berries, fruits, herbs, honey.

In the traditional method, either dried rye bread or a combination of rye flour and rye malt is used.

The exact origins of kvass are unclear and whether it was invented by Slavic people or any other Eastern European ethnicity is unknown. Kvass has existed in the northeastern part of Europe, where the grain production might have been insufficient for beer to become a daily drink. The first written mention of kvass is found in the "Primary Chronicle", describing the celebration of Vladimir the Great's baptism in 996 when kvass along with mead and food was given out to the citizens of Kiev. Kvass making remained a daily household activity well into the 19th century.

In the second half of the 19th century, with military engagement, increasing industrialization and large-scale projects, such as the construction of the Trans-Siberian Railway, that created a growing need to supply large numbers of people with foodstuff for extended periods of time, commercial kvass producers began appearing in the Russian Empire.

In the 1890s, the first scientific studies into the production of kvass were conducted in Kyiv, and in the 1960s, the commercial mass production technology of kvass was developed by chemists in Moscow.